How do I extend this mergeWords function to any number of strings?

Question

Following is the mergeWords function.

mergeWords [] [] = []
mergeWords [] (y:ys) = y:'\n':(mergeWords [] ys)
mergeWords (x:xs) [] = x:'\n':(mergeWords xs [])
mergeWords (x:xs) (y:ys) = x:y:'\n':(mergeWords xs ys)

If applied on mergeWords "hello" "world" it gives

"hw\neo\nlr\nll\nod\n"

I can't figure out how to extend this to list of strings. Like applying it to 3 strings should first take first character of each of the strings and then put a '\n' and then the second character and so on.

Answer 1

Sounds reasonably easy if you do it in steps:

cutWords :: [String] -> [[String]]    -- ["ab", "cd", "e"] -> [["a", "c", "e"], ["b", "d"]]
concatWord :: [String] -> String       -- ["a", "c", "e"] -> "ace\n"
concatWords :: [String] -> String    -- use mergeWord on all of them

The most interesting part is of course the cutWords part. What you want there is a zip-like behaviour, and for that it'll help if we "safe" tail and head:

head' (x:xs) = [x]
head' "" = ""

tail' (x:xs) = xs
tail' "" = ""

Now we can implement our cutWords, making sure we stop in time:

cutWords xs = heads : rest
  where
    heads = map head' xs
    tails = map tail' xs
    rest = if any (/= "") tails then cutWords tails
                                else []

Then the remaining part is trivial:

concatWord word = concat word ++ "\n"
concatWords words = concatMap concatWord word